Spontaneous electrophoretic and chromosomal variability in callus cultures and regenerated plants of celery

Reciprocal sexual crosses were performed to produce plants heterozygous at four nuclear loci, two of which coded for phosphoglucomutase (PGM) and shikimic acid dehydrogenase (SDH) isozymes and could be visualized in petiole callus tissue. Clones of one reciprocal always exhibited the heterozygous phenotype at both isozyme loci, but 25.8% of clones of the other exhibited loss of the fast migrating electromorph at the PGM locus while remaining 100% heterozygous at the SDH locus. No reversion to heterozygosity was observed and the variant phenotype was transmitted to regenerated plantlets. All regenerates were karyologically and developmentally abnormal, and studies of the inheritance of this variability were therefore not possible. Epigenetic change, point mutation, and somatic recombination were ruled out as possible causes due to gross incongruencies with the phenomenon. No consistent differences between normal and variant clones were detected with respect to chromosome number, structure, and anomalous disjunction.